1. Field of the Invention
This invention relates to a pulse wave radar device using the millimeter wave or submillimeter wave band, or in particular to a pulse wave radar device having an improved detection ability for the reflected wave from an object at a short distance.
2. Description of the Related Art
In recent years, a radar DEVICE has come to be mounted on an automotive vehicle for the purpose of collision prevention and auto cruising. In this in-vehicle radar DEVICE, the reciprocating distance to an object is determined by multiplying the velocity of light by the time from the transmission of the transmitting pulse wave to the receiving of the reflected wave from the object. Thus, the distance to the object is calculated by measuring the time required from the transmission of the transmitting pulse wave to the receiving of the reflected wave from the object.
This pulse wave radar device makes the preparation for receiving the receiving pulse wave reflected from the object at the shortest distance after transmitting the transmitting pulse wave. The in-vehicle pulse wave radar device is required to detect an object within a wide range of several tens of cm to several tens of m, and therefore the receiving circuit requires an amplifier circuit having a wide dynamic range from the receiving pulse wave of a large signal to the receiving pulse wave of a small signal.
In the prior art, as the receiving circuit of the pulse wave radar device requiring a wide dynamic range, an automatic gain control (AGC) circuit is used. The configuration of the conventional pulse wave radar device using the AGC circuit is shown in FIG. 1 (see Japanese Patent Application Laid-Open No. 6-174826, for example).
In FIG. 1, reference numeral 11 denotes a pulse generating circuit, 12 an oscillator, 21 a mixing circuit, 22 a power amplifier circuit, 23 a transmitting antenna, 31 a receiving antenna, 82 a pre-amplifier circuit, 83 a mixing circuit, 84 an intermediate frequency amplifier circuit, 85 a detection circuit, and 86 an AGC circuit. In the mixing circuit 21, the pulses from the pulse generating circuit 11 are modulated by the oscillation wave from the oscillator 12, after being amplified in the power amplifier circuit 22, emitted as a transmitting pulse wave from the transmitting antenna 23.
In the mixing circuit 83, the receiving pulse wave received by the receiving antenna 31 is demodulated by the oscillation wave from the oscillator 12, and amplified by the intermediate frequency amplifier circuit 84. During the amplification, the output of the detection circuit 85 is controlled by negative feedback through the AGC circuit 86 to produce a constant value.
Also, the variable attenuation circuit using the PIN diode as shown in FIG. 2 is used for a variable gain amplifier circuit. In FIG. 2, reference characters R71 to R76 denote resistors, C71 to C74 denote capacitors, and D71 to D73 denote PIN diodes.
The PIN diodes D71 to D73 are DC blocked by the capacitors C71 to C74, and the resistance value of the PIN diodes D71 to D73 is variably controlled by the bias voltage from a DC power supply. Once the resistance value of the PIN diodes is variably controlled, the attenuation amount of the variable attenuation circuit is changed, and thus the amplification degree of the variable gain amplifier circuit is controlled.